Dissociative excitation of CH4 by collisions with helium active species

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Abstract
Dissociative excitation of CH4 by collisions with He(2 3S), He+, and He+2 has been studied by observing CH(A 2Δ–X 2Πr, B 2Σ−–X 2Πr, and C 2Σ+–X 2Πr) and H (Balmer) emissions in the flowing afterglow and beam apparatus. The effect of SF6 addition into the He afterglow indicated that the formation of CH(A,B,C) in the flowing afterglow proceeds through both the primary He(2 3S)/CH4 reaction and a secondary electron–ion recombination reaction through the He+/CH4 and/or He+2/CH4 reactions. The emission rate constants of A–X, B–X, and C–X of CH, Hα, and Hβ produced from the He(2 3S)/CH4 reaction were determined to be 5.6, 1.5, 0.011, 0.46, and 0.072×10−13 cm3 s−1 in the beam experiment, respectively. The nascent vibrational distribution of CH(A) from He(2 3S)/CH4 was determined as N0:N1=100:37±5. The rotational distributions were expressed by single Boltzmann temperatures of 3200±200 and 2600±300 K for v’=0,1 of CH(A) and 3300±200 K for v’=0 of CH(B).